1. Field of the Invention
The present invention relates to a method for detecting an etching endpoint in a plasma etching process, and an etching apparatus and an etching system using such a method.
2. Description of Related Art
FIG. 1 is a block diagram and a model diagram showing the configuration of a conventional etching apparatus. In FIG. 1, numeral 1 designates a vacuum processing chamber, in which an upper electrode 2a and a lower electrode 2b are arranged in opposed relation to each other. The side wall of the vacuum processing chamber 1 has a detection window 4, outside of which an actinometer 21 and an endpoint detector 22 are arranged.
The lower electrode 2b has a semiconductor substrate 3 arranged thereon. A predetermined process gas is introduced into the vacuum processing chamber 1, and a high-frequency voltage is applied between the upper electrode 2a and the lower electrode 2b from a high-frequency power source 8 thereby to subject the semiconductor substrate 3 to plasma etching.
In fabricating a semiconductor device, it is important to etch exactly a predetermined amount. Unless the amount of etching is proper, such problems as an increase in the number of defective products may be imposed. In view of this, an etching endpoint is detected by the endpoint detector 22 for finishing the etching process. An etching endpoint is conventionally detected in the following manner. The plasma light generated due to reaction products during the plasma etching process is reduced by removal of the object etched. The change in amount of light is measured by the actinometer 21 through the detection window 4, and an etching endpoint is detected by the endpoint detector 22.
However, the detection window 4 faces the inside of the vacuum processing chamber 1, and is exposed all the time to the reaction products generated during the etching process. Hence, with the lapse of time, reaction products are progressively deposited on the detection window 4. Then the amount of light received by the actinometer 21 outside of the detection window 4 is reduced by an amount more than what would otherwise be reduced by an etching process alone. The result is variations of the signal intensity outputted from the actinometer 21, leading to a displacement of the etching endpoint detected, or making the detection impossible. It is therefore necessary to correct the output signal of the actinometer 21 by gain adjustment of the signal intensity or the like means.
Japanese Patent Application Laid-Open No.3-14229(1991) discloses an endpoint detection apparatus comprising an automatic bias regulator for accumulating the applied time of high-frequency power and automatically regulating the bias of an actinometer in accordance with the accumulated time. Also, Japanese Patent Application Laid-Open No.62-165920(1987) describes an etching endpoint judging apparatus comprising a sensitivity regulation circuit for regulating to a set value the amount of light reduced due to the clouding of the detection window.
In the former apparatus, it is difficult to attain correspondence between the accumulated time and the automatic bias regulation in a manner to satisfy the types of the process gas, the object to be etched and the apparatus. The latter disclosure, on the other hand, poses the problem of requiring a sensitivity regulation circuit to be separately provided.
Further, an apparatus is conceivable having such a function that the original waveform obtained by plotting the output signal of the actinometer 21 is corrected by primary differentiation and secondary differentiation. Mere differentiation, however, develops conspicuous fluctuations in the original waveform and reduces the detection accuracy of an etching endpoint.
Furthermore, in cases where an amount of the deposited reaction products on the detection window is large and where an exposed area of an object (wafer) to be etched is small, an etching process undergoes only a slight change in the amount of light between the initial and final periods thereof. When the change is slight, detection of an endpoint is very difficult and the signal is required to be amplified.